Currently, a printer is used in financial institutions for printing images on a booklet such as a bankbook and a ledger sheet such as a bank statement. Typically, a dot printer is used to print images on the booklet. The booklet is inserted into the dot printer by a user and the printing is performed thereon. On the other hand, a thermal printer is typically used to print the ledger sheet, and a sheet stored in the thermal printer is used to print the ledger sheet. One type of a printer has both a dot printing unit and a thermal printing unit, and has a single sheet conveyance path that is shared by the dot printing unit and the thermal printing unit to make the printer smaller. Another type of a printer that prints images on the booklet has a scanner (image sensor) that scans a surface of the booklet, for example to detect the page number of a sheet of the booklet on which the printing is performed.
Generally, in order to properly detect an image on a scanned surface of a sheet, a scanner needs to periodically perform a shading correction. Usually, a reference plate is used to reset detection values for white during the shading correction. However, the reference plate may become an obstacle to making the size of the printer small.